Flow domes of Miocene rhyolite
intrude a thick sequence of basaltic andesite in the Sheep Creek Range and
southern part of the Shoshone Mountains, are overlain by olivine basalts, and
are about 13.9 Ma (McKee and Silberman, 1970). These domes form high spires in
the center and curved ridges along the margins of the range where steeply
dipping flow planes are strongly developed (Stewart and McKee, 1977). Flows,
flow breccia, welded and non welded tuffs, and intrusives of rhyolitic
composition are present within the domes.

Rhyolitic and dacitic flows and
composite domes are exposed to the north of the Sheep Creek Range within the
quadrangles surrounding the Midas and Wilson Creek areas (Coats, 1985). Both
flows and domes of silicic rhyolite flows, and welded tuffs with 5 to 30 percent
phenocrysts of quartz, sanidine, plagioclase, and minor hornblende and biotite
are present (Coats, and others, 1985). In the Midas area these gray to brown or
red-gray rhyolite, rhyodacite, and dacite flows and domes are as much as 2000
feet in thickness. Farther north in the northern portion of the Hat Peak
Quadrangle a mass of porphyritic fayalite-ferro-augite rhyolite about 8 miles
across was named the Circle Creek Rhyolite by Coats (1968). This rhyolite has a
vitric groundmass with phenocrysts of quartz, sanidine, oligoclase, ferrroaugite,
fayalite, apatite, zircon, magnetite and illmenite as well as chalcedony lined
nodules (Coats, 1985).

K-Ar dating of sanidine from the
Circle Creek Rhyolite shows it is 11.6 +/- 0.5 Ma (Coats, 1968). Similar
rhyolite with sanadine, tridymite, olivine and clinopyroxene phenocrysts occurs
south of Wendover and has been dated from sanidine as 11.6 +/- 0.4 Ma (Coats,
1985). Rhyolite with phenocrysts of quartz, plagioclase, sanidine, hornblende,
biotite, and perrierite are informally called the
rhyolite of Coldspring
Mountain in the Mountain City Quadrangle in northern Elko County, and have been
dated on sanidine as 14.6 +/- 0.9 Ma (Coats, 1985; McKee and others, 1976).

Three breccia pipes or volcanic
vents about 1 mile across are present just west of Mount Lewis in the northern
Shoshone Range. Coarse and fine-grained breccia and foundered Paleozoic and
Tertiary intrusive blocks are present within the pipes. The coarse breccias
contain angular fragments of rock from a few inches to 100 feet in length, while
the fine breccias consist of subrounded to angular Paleozoic and Tertiary
volcanic rock fragments up to a few inches across (Gilluly and Gates, 1965).
These breccia pipes are cut by discontinuous, flow banded rhyolite or quartz
latite dikes from 10 to 100 feet thick.

Miocene rhyolite flows and plugs
which represent slightly dissected domes are also present along the eastern
flank of the Cortez Range along Willow and Horse Canyon Creeks in T. 26 N., R.48
E. These rhyolites are lithoidal and vesicular with small amounts of sanidine
and biotite phenocrysts as much as 3 mm in length in a devitrified groundmass
(Gilluly and Masursky, 1965).

In the northern Reveille Range,
rhyolitic and rhyodacitic plugs, dikes and sills of Miocene age are exposed in
several small areas (Ekren, Rogers, and Dixon, 1973). Most exposures are flow
layered and are pink to brown, fresh to intensely propylitically-altered rocks
with about 10 percent phenocrysts of alakali feldspar, plagiocalse and biotite,
with minor quartz, hornblende and clinopyroxene.

At the southern end of Monitor
Valley and in local patches overlying Oligocene tuffs in the Monitor Range,
rhyolite lavas and flow domes of probable Miocene age are exposed (Kleinhampl
and Ziony, 1985). These gray, brown and yellow rhyolites are massive to flow
banded, with pumaceous and scoriaceous margins and bases, and are locally 700
feet, and up to 1,500 feet in thickness at Hat Peak. Kleinhampl and Ziony (1985)
suggest that these flows, as well as discontinuous flows in the southern Toquima
Range such as the Kanrohat Rhyolite, range in age from 26 to 16 Ma, making them
coeval with flows both in Sand Springs Valley and the Oddie Rhyolite (16 Ma) in
the Tonopah and Toquima Ranges area. The Oddie Rhyolite is a pink to purple
fine-grained and locally brecciated and porphyritic flow rock with small quartz,
biotite, and feldspar phenocrysts.

Massive to flow banded devitrified,
white to gray or red Miocene rhyolite flows up to 500 feet thick are present in
various local patches in the Hot Creek Range. These flows are best exposed in
the Tybo area of the Hot Creek Range where they intrude the Miocene
Tuff of Kiln
Canyon (Quinlivan and Rogers, 1974). These flows are probably coeval with the
rhyolite of Big Sand Springs Valley (Kleinhampl and Ziony, 1985).

In Big Sand Springs Valley along
the western flank of the Pancake Range, large masses of Miocene rhyolite, named
the Rhyolite of Big Sand Springs, are dated at 25.8 +/- 1.3 Ma. (Quinlivan and
others, 1974). These purple-gray rhyolites are massive to flow-banded with a
thin and discontinuous basal vitrophyre underlain locally with flow breccia, and
10 to 100 feet of basal with pumiceous air-fall tuff. These flows were
apparently erupted along ring fractures bordering the Lunar Lake Caldron and are
up to 1,300 feet in thickness (Quinlivan and others, 1974).

In the Carlin-Pinon area the
Palisade Canyon Rhyolite is exposed as prominent cliffs on both sides of the
Humboldt River southwest of Carlin (Smith and Ketner, 1976). This series of
red-brown flow banded rhyolites was named by Regnier (1960) for Palisade Canyon
in T. 31 N., R. 52 E. Individual flows are up to 200 feet thick and contain
phenocrysts of sanidine, quartz, biotite, and pigeonite in a spherulitic
groundmass; the base of each flow is black and glassy (Regnier, 1960; Smith and
Ketner, 1976). The Palisade Canyon is exposed at the northern end of Pine Valley
near the northern tip of the Cortez Mountains, where it is about 800 feet thick
and forms a wedge of flows in the Humboldt Formation. The unit is Upper Miocene
in age and gives a whole rock age of 15.0 +/- 1.0 Ma (Smith and Ketner, 1976).

To the north in Elko County the
Miocene Jarbidge Rhyolite (Coats, 1964) is one of the most widespread volcanic
units. This rhyolite is exposed at the northern end of the East Humboldt Range
and at Secret Pass between the East Humboldt and Ruby Ranges, along the north
fork of the Humboldt River east of the Independence Mountains in the North Fork, Mahala Creek East, and Devils Armchair Quadrangles, to the north in the Bull
Run, northern Wild Horse, eastern and southern Mountain City, western Rowland,
Jarbidge, Marys River Basin Northwest and Northeast, Stags Mountain, Hanks
Creek, and Hot Springs Quadrangles, and in the Elk Mountains (Coats, 1985). K-Ar
radiometric dates on sanadine within the rhyolite in the Jarbidge and Rowland
Quadrangles show that the unit ranges in age from 16.8 +/- 0.5 Ma to 15.4 Ma
(Coats and others, 1977).

The extensive Jarbidge Rhyolite
forms large extrusive masses of pervasively altered and recrystallized flows and
very minor air fall and welded tuffs, and small dikes and vitrophyres, that form
sections up to 2000 feet in thickness frequently cut by numerous high-angle
faults (Coats, 1964, 1985). Small amounts of coaly material are locally
interbedded with the rhyolite (Coats, 1985). These rhyolites are pale blue gray
to yellowish-gray and weather to a red, purple, or green where propylitically
altered. They characteristically contain abundant rounded and corroded or
euhedral quartz phenocrysts up to 5 mm in diameter and smaller and less abundant
sanidine and oligoclase phenocrysts (Coats, 1964). Biotite, hornblende and
pigeonite are present locally and accessories in most rhyolites and include
zircon, apatite, ilmenite, magnetite and garnet (Coats, 1985). The Jarbidge
Rhyolite has been altered propylitically with the development of chlorite and
epidote, and replacement of groundmass plagioclase and sanidine phenocrysts by
adularia and clay minerals (Coats, 1985). It is the host for gold and silver
bearing veins which produced about 10 million dollars worth of gold from 1910 to
1950 in the Jarbidge district (Coats and others, 1977).

Also exposed in the Jarbidge
Mountains area are several Miocene, gray to black dacitic flows, dikes,
intrusive domes and vitrophyres including the Gods Pocket,
Pole Creek, and
Robinson Creek Dacites (Coats, 1964). These masses are less than 1.5 miles in
lateral extent. The Robinson Creek Dacite intruded the Jenny Creek Tuff in the
Jarbidge Quadrangle and is 200 feet thick at its maximum. The Pole Creek Dacite
is slightly older than the Robinson Creek Dacite and is mainly composed of gray
to black vitrophyre up to 900 feet thick (Coats, 1964). The Gods Pocket Dacite,
named for Gods Pocket Peak, is exposed in the Jarbidge Quadrangle and Jarbidge
wilderness area where it overlies the Cougar Point Tuff and underlies the
Jarbidge Rhyolite, reaching a maximum thickness of 930 feet. The Gods Pocket
dacites are dense, pale-gray porphyritic flows with relatively large phenocrysts
of andesine, sanidine, quartz, pigeonite, and rare augite and hornblende (Coats,
1985).

Miocene rhyolites in the northern
portion of the Snake Mountians are about 700 feet thick and have been dated at
12.6 and 13.7 +/- 0.3 Ma (Gardner, 1968). The lower portion of the rhyolite is
commonly about 10 feet of porphyritic gray flow banded rhyolite, and is overlain
by black and greenish gray massive rhyolite with 5 to 30 percent phenocrysts of
quartz, sanadine, oligoclase, augite, microcline, and biotite. Individual units
are commonly about 100 feet in thickness (Gardner, 1968).

Miocene rhyolites in the 17 to 25
Ma age range are also exposed in Lincoln County within the eastern and southern
Wilson Creek Range near the Mount Wilson and Parsnip Peak volcanic centers and
in the central Fairview and Quinn Canyon Ranges. These flows are laminated and
layered, pink to bluish-gray, crystal poor rhyolite with some crystals of alkali
feldspar, quartz, and biotite (Ekren and others, 1977).